The present invention generally relates to a smart antenna array technology used in a cellular mobile communication system, and more particularly to a method that can improve smart antenna array coverage.
In a cellular mobile communication system using a smart antenna array, the smart antenna array is built into a radio base station, in general. The smart antenna array must use two kinds of beam forming for transmitting and receiving signals: one kind is the fixed beam forming, while another is the dynamic beam forming. The fixed beam forming, such as omnidirectional beam forming, strip beam forming or sector beam forming, is mainly used for transmitting omnidirectional information, such as broadcasting, paging etc. The dynamic beam forming is mainly used for tracing subscribers and transfers a subscriber""s data and signaling information, etc. to a specific user.
FIG. 1 shows a cell distributing diagram of a cellular mobile communication network. Coverage is the first issue to be considered when designing a cellular mobile communication system. In general, a smart antenna array of a wireless base station is located at the center of a cell, as shown by the black dots 11 in FIG. 1. Most cells have normal circle coverage, as shown by 12. Some cells have non-symmetric circular coverage, as shown by 13, and xe2x80x9cstripxe2x80x9d coverage, as shown by 14. The normal circle coverage 12, non-symmetric circular coverage 13 and strip coverage 14 are overlapped for non-gap coverage.
It is well known that a power radiation diagram of an antenna array is determined by the parameters such as: geometrical arrangement shape for antenna units of the antenna array, characteristics of each antenna unit, phase and amplitude of radiation level of each antenna unit, etc. When designing an antenna array, in order to make the design one that can be commonly used, the design is taken under a relatively ideal environment, which includes free space, equipment works normally, etc. When a designed antenna array is put in practical use, the real power coverage of the antenna array will certainly be changed because of different installing locations and positions, different landforms and land surface features, different building heights and different arrangements of antenna units, etc.
FIG. 2 (part of FIG. 1) shows a difference of an expected coverage 21 (normal circle) and a real or actual coverage 22, as such real coverage is caused because of different landforms and land surface features, etc. The real coverage can be measured at a cell""s site. It is possible that every cell has this kind of difference, so unless adjustments are made at a cell""s site, real coverage of a mobile communication network may be very bad. Besides, there is a need to reconfigure an antenna array when an individual antenna unit of the antenna array does not work normally or coverage requirement has been changed, at this time the coverage of the antenna array must be adjusted in real time.
The principle of the adjustment is: based on fixed beam forming for omnidirectional coverage of a cell, a smart antenna array implements dynamic beam forming (dynamic directional radiation beam) for an individual subscriber.
For formula (1): A(xcfx86) represents the shape parameter of the expected beam forming, (i.e., the needed coverage), wherein 4) represents polar coordinate angle of an observing point, and A(xcfx86) is the radiation strength in the xcfx86 direction, with same distance.
Shape Parameter Of The Expected Beam Forming=A(xcfx86)xe2x80x83xe2x80x83(1) 
Suppose there are N antennas for a smart antenna array, wherein any antenna n has a position parameter D(n), a beam forming parameter W(n) and an emission power P in angle xcfx86 direction, then the real coverage is represented by formula (2):                               P          ⁡                      (            φ            )                          =                              "LeftBracketingBar"                                          ∑                                  n                  =                  1                                N                            ⁢                                                f                  ⁡                                      (                                          φ                      ,                                              D                        ⁡                                                  (                          n                          )                                                                                      )                                                  xc3x97                                  W                  ⁡                                      (                    n                    )                                                                        "RightBracketingBar"                    2                                    (        2        )            
Wherein the form of the function f(xcfx86,D(n)) is related with the type of a smart antenna array.
In a land mobile communication system, taking into account two dimensional coverage on a plane is enough, in general. When dividing antennas in an arrangement, there are linear arrays and a ring arrays. A circular array can be seen as a special ring array (refer to China Patent 97202038.1, xe2x80x9cA Ring Smart Antenna Array Used For Radio Communication Systemxe2x80x9d). In a cellular mobile communication system, when implementing sector coverage, a linear array is generally used, and when implementing omnidirectional coverage, a circular array is generally used. In the present invention, a circular array is used as an example.
Suppose it is a circular array, then D(n)=2xc3x97(nxe2x88x921)xc3x97xcfx80/N;
f(xcfx86,D(n))=exp(jxc3x972xc3x97r/xcexxc3x97xcfx80xc3x97cos("PHgr"xe2x88x92D(n)) (find exponent). 
Wherein r is the radius of a circular antenna array and xcex is the working wavelength. FIG. 3, for example, shows a power directional diagram of an omnidirectional beam forming for a normal circle antenna array with 8 antennas. Squares of digits 1.0885, 2.177, 3.2654, shown in FIG. 3, represent power.
Using a minimum mean-square error algorithm, the mean square error xcex5 in formula (3) is the minimum one:                     ϵ        =                              1            K                    ⁢                                    ∑                              i                =                1                            K                        ⁢                                                            "LeftBracketingBar"                                                                                    P                        ⁡                                                  (                                                      φ                            i                                                    )                                                                                            1                        /                        2                                                              -                                          A                      ⁡                                              (                                                  φ                          i                                                )                                                                              "RightBracketingBar"                                2                            xc3x97                              C                ⁡                                  (                  i                  )                                                                                        (        3        )            
In formula (3), K is the number of sampling points, when using an approximation algorithm; and C(i) is a weight. For some points, if the required approximation is high, then C(i) is set larger, otherwise C(i) is set smaller. When required approximations for all points are coincident, C(i) will be set as 1, in general.
Further, considering that transmission power of every antenna unit is limited, when taking the amplitude of W(n) to represent the transmission power of an antenna unit, and setting the maximum transmission power of each antenna unit as T(n), the limited condition can be expressed as:
|W(n)|xe2x89xa6T(n)1/2xe2x80x83xe2x80x83(condition 1) 
Obviously, to find out an optimal value of the transmission power within the limit for every antenna unit, in general it only can be solved by selection and exhaustion of unsolved W(n) accuracy, except for some special situations which can be directly solved by a formula. Nevertheless, when using such an exhaustive solution, the calculation volume is very large and has an exponential relationship with the number of antenna units N. Although, the calculation volume can be decreased by gradually raising the accuracy and decreasing the scope of the value to be solved, but even only to solve for this sub-optimal value, the calculation volume is still too large.
In order to effectively improve smart antenna array coverage, a method to improve smart antenna array coverage has been designed. The improvement includes having the real coverage of an antenna array approach the design coverage; and when part of an antenna unit is shut down because of trouble, the antenna radiation parameter of other normal working antenna units can be immediately adjusted to rapidly recover the cell coverage.
The purpose of the invention is to provide a method, which can adjust parameters of antenna units of an antenna array according to a practical need. With this method, an antenna array has a specific beam forming satisfying requirement, and the emission power optimal value of each antenna unit can be rapidly solved within a limit to obtain a local optimization effect.
The method of the present invention is one kind of baseband digital signal processing methods. The method changes the size and shape of the coverage area of a smart antenna array, by adjusting parameter of each antenna (excluding those shut down antennas) of the smart antenna array, to obtain a local optimization effect coinciding with requirement under minimum mean-square error criterion. The specific adjusting scheme is that according to a difference of size and shape between coverage required in engineering design and actually realized coverage, an antenna""s radiation parameters are adjusted by a method of step-by-step approximation under the minimum mean-square error criterion, in order to make the actual coverage of an antenna array approximate the engineering design requirements under locally optimized conditions.
According to the present invention, adjusting the beam forming parameter W(n) for each antenna unit n of an N antenna array, according to actual situations, further comprises:
A. setting an accuracy of W(n) to be solved, i.e. an adjusting step length;
B. setting initial values, including: an initial value W0(n) of beam forming parameter W(n) for antenna unit n; an initial value co of minimum mean-square error xcex5, a counting variable for recording the minimum adjustment times; an adjustment ending threshold value M and a maximum emission power amplitude T(n) for antenna unit n;
C. entering a loop for W(n) adjustment which comprises: generating a random number; deciding a change of W(n) by the set step length and calculating a new W(n); when deciding the absolute value of W(n) being less than or equal to T(n)1/2, calculating the minimum mean-square error xcex5; when xcex5 is greater than or equal to xcex50; keeping the xcex5 and increment the counting variable by 1;
D. repeating the step C until the counting variable is greater than or equal to the threshold value M, ending the adjusting procedure and getting the result; recording and storing the final W(n), and replacing the xcex50 with the new xcex5.
When comparing xcex5 and xcex50 in the step C, if is less than xcex50, then the calculation result W(n) of this time adjustment is recorded and stored, the xcex50 is replaced with the new calculated xcex5 and the counting variable is reset to zero.
The adjusting step length can be fixed or varied. If the adjusting step length is varied, then setting a minimum adjusting step length is also included during the setting of initial values. When the counting variable is greater than or equal to the threshold value M, but the adjusting step length is not equal to the minimum adjusting step length, the adjusting step length is continually decreased and the adjusting procedure of W(n) is continued.
The adjusting procedure ending conditions further include a preset adjustment ending threshold value xcex5xe2x80x2, and when xcex5 less than xcex5xe2x80x2, the adjustment is ended.
The number of the initial value W0(n) is related to the number of antenna units, which comprise the smart antenna array.
When setting the initial value W0(n) of W(n), W0(n) is set to zero for antenna units of the smart antenna array that are shut down and W(n) for the shut down antenna units will not be adjusted in the successive adjusting loop.
The minimum mean-square error xcex5 is calculated by the following formula:       ϵ    =                  1        K            ⁢                        ∑                      i            =            1                    K                ⁢                                            "LeftBracketingBar"                                                                    P                    ⁡                                          (                                              φ                        i                                            )                                                                            1                    /                    2                                                  -                                  A                  ⁡                                      (                                          φ                      i                                        )                                                              "RightBracketingBar"                        2                    xc3x97                      C            ⁡                          (              i              )                                            ,
Wherein P(xcfx86i) is an antenna unit""s emission power when the beam forming parameter of the antenna unit is W(n) and the directional angle is xcfx86, and P(xcfx86i) is related to the antenna array type; A(xcfx86i) is the xcfx86 directional radiation strength with equal distance and the expected observation point having phase xcfx86 for polar coordinates; K is the number of sample points when using the approximate method and C(i) is a weight.
The setting of an accuracy of W(n) to be solved, i.e. an adjusting step length, comprises:
Setting the stepping change of the real part and an imaginary part for a complex number W(n), respectively; or setting the stepping change of an amplitude and a phase for a polar coordinates W(n), respectively;
when using the stepping change of a real part and an imaginary part for a complex number W(n), the new W(n) is calculated by the formula: WU+1(n)=WU(n)+xcex94WU(n)=IU(n)+(xe2x88x921)L1Uxcex94IU(n)+j*└QU(n)+(xe2x88x921)LOUxcex94QU(n)┘, wherein xcex94IU(n) and xcex94QU(n) are the adjusting step length of the real part IU(n) and imaginary part QU(n), respectively; L1U and LQU decide adjusting direction of the real part IU(n) and imaginary part QU(n), respectively; their values are decided by a generated random number;
when using the stepping change of an amplitude and a phase for a polar coordinates W(n), the new W(n) is calculated by the formula: WU+1(n)=WU(n)*xcex94WU(n)=AU(n)*xcex94AU(n)(xe2x88x921)LAU*ej*[xcfx86U(n)+(xe2x88x921)LUxcfx86xcex94xcfx86U(n)], wherein xcex94AU(n) and xcex94xcfx86U(n) are the adjusting step length of the amplitude AU(n) and phase xcfx86U(n), respectively; LAU and Lxcfx86U decide adjusting direction of the amplitude AU(n) and phase xcfx86U(n), respectively, their value are decided by a generated random number;
the U is the Uth adjustment and U+1 is the next adjustment.
The method of the invention concerns the case that when a radio base station uses a smart antenna array for fixed beam forming of omnidirectional coverage, the smart antenna array coverage can be effectively improved. The coverage size and shape of a smart antenna array is changed by adjusting the parameters of each antenna unit of the antenna array in order to obtain a local optimal effect of coincident requirement under the minimum mean-square error criterion.
The method of the invention is that according to a difference of size and shape between coverage required in engineering design and actually realized coverage, an antenna""s radiation parameters are adjusted by a method of step-by-step approximation under the minimum mean-square error criterion, in order to make the actual coverage of an antenna array approximate the engineering design requirement under local optimization conditions.
One application of the method is at the installation site of a smart antenna array; where the coverage size and shape of a smart antenna array can be changed by adjusting the parameters of each antenna unit of the smart antenna array to obtain an omnidirectional radiation beam forming which closely approximates an expected beam forming shape and has a local optimization results for coinciding with engineering design requirements. Another application of the method is that when one or more of the antenna units in a smart antenna array are not normal and have been shut down, antenna radiation parameters of the remaining normal antenna units can be immediately adjusted by the method to immediately recover omnidirectional coverage for the cell.